Analogs of MPTP, substituted in the phenyl ring with chlorine or hydroxy groups were synthesized. The synthesis also afforded peperidine and pyridine congeners of MPTP. The compounds were evaluated in vitro as inhibitors of dihydroteridine reductase. The most active inhibitor was 4-(3 feet, 4 feet-dihdroxyphenyl)-pyridine. The N-oxide of MPTP may constitute an intermediate in the conversion of MPTP to 4-phenylpyridinium salts, detected as major metabolites of MPTP in the brain of monkeys. The N-oxide of MPTP converted with anhydrides in vitro into dihydro intermediates which smoothly aromatized to pyridinium salts with air and Pt catalyst. The dihydro intermediates could also be aromatized with the N-oxide of MPTP. The N-oxide of MPTP also oxidizes in vitro dopamine into 6-hydroxydopamine. It will now be explored whether the N-oxide of MPTP is being formed in animals or man.